Titanium-based heat exchangers and methods of manufacture

ABSTRACT

Oxidation protection of a titanium heat exchanger is provided by a titanium aluminide or solgel coating. The coating protects bare titanium and brazed surfaces of the heat exchanger.

REFERENCE TO CROSS-RELATED APPLICATION

[0001] This is a continuation-in-part of U.S. Ser. No. 08/865,905, filedon May 30, 1997, now pending.

BACKGROUND

[0002] The invention relates to heat exchangers. More specifically, theinvention relates to oxidation protection of titanium-based heatexchangers.

[0003] Certain heat exchangers used in aircraft environmental controlsystems are exposed to temperatures exceeding 400° C. These heatexchangers are typically made of stainless steel, which can withstandthe high temperatures.

[0004] It would be desirable to use titanium heat exchangers instead ofstainless steel heat exchangers. Titanium and its alloys have a lightweight and can provide a weight reduction of up to 40% over comparablestainless steel heat exchangers. The weight reduction results in betterfuel efficiency and lower aircraft operating costs.

[0005] However, titanium is not used for high temperature heat exchangerapplications because the titanium exhibits a propensity to rapidlyoxidize, (over a couple of hours), at the required operatingtemperatures. Oxidation of titanium results in a reduction in ductilityand then strength, and a deterioration in structural integrity. Repeatedthermal cycling at temperatures between ambient temperature and around400° C. (and higher) causes the titanium to crack. Cross contaminationof fluids can occur and lead to life-threatening conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is an illustration of a core of a titanium heat exchanger.

[0007]FIG. 2a is an illustration of a first method of applying aprotective coating to the titanium heat exchanger.

[0008]FIG. 2b is an illustration of a second method of applying aprotective coating to the titanium heat exchanger.

[0009]FIG. 3 illustrates a titanium aluminide protective coating.

[0010]FIG. 4 Illustrates the importance of the transformationtemperature on the oxidation resistance of the coating.

DETAILED DESCRIPTION

[0011] Reference is made to FIG. 1. A heat exchanger core 10 includes aplurality of flat parallel plates 11 that are separated by spacers orfins 12. The plates 11 and fins 12 define first and second fluid flowpassageways 13 and 14. Dimensions of the passageways 13 and 14 aretypically between about 0.5 mm and 2 mm square, and may be about 400 mmlong. The passageways 13 and 14 may be designed to promote turbulence inthe fluid flow in order to improve heat transfer by the avoidance ofboundary layers. Although not shown as such, the passageways 13 and 14of the heat exchanger core 10 may have wavy shapes or other complexshapes that create turbulence.

[0012] The plates 11 and fins 12 are made of a titanium-based metal.Making the heat exchanger core 10 of titanium offers a 40% weight savingover stainless steel or superalloys that are currently used. Because ofits complex geometry the material used to make the heat exchanger mustbe formable into complex geometries, preferably at room temperature.Titanium, alpha, alpha-plus-beta and beta titanium alloys are preferred.

[0013] The fins 12 are bonded to the plates 11 by brazing. Duringfabrication, the plates 11 and fins 12 and brazing filler metal arestacked to form a stacked assembly. The stacked assembly is then heatedto form a brazed and unified core 10. In addition to bonding the plates11 to the fins 12, the braze maintains integrity of the fluidpassageways 13 and 14. Most of the plates 11 and fins 12 are coated withthe braze alloy. A large fraction of the surface of the heat exchangercore 10 is braze clad and therefore has a varying chemistry and surfacefinish.

[0014] Typical brazes for titanium and its alloys contain less than 50%titanium, with 20% being usual. Other typical metals in the braze arecopper, silver, nickel and zirconium. Some titanium brazes contain notitanium (e.g., a braze including Ag 82% Pd 9% Ga 9%) and many onlycontain trace amounts of titanium (e.g., a braze including Ag 59% Cu27.25% In 12.50% Ti 1.25%). Thus a range of alloys are used to brazetitanium, and after brazing these alloys will have a composition rangingfrom that of the braze to that of the titanium alloy.

[0015] Manifolds and end plates (not shown) are typically welded to theheat exchanger core 10. The manifolds and end plates are typically notbraze coated. Therefore, the manifolds and ends of the fins are usuallybare titanium.

[0016] The completed heat exchanger has a range of surfaces. Some ofthese surfaces result from individual manufacturing processes such ascasting and rolling. Other surfaces are brazed and welded. Some of thesesurfaces are inside the heat exchanger and, once formed, cannot bevisually inspected or prepared for coating.

[0017] A protective coating is applied to exposed surfaces of the heatexchanger core 10, the manifolds and the end plates. The coating may beeither a titanium aluminide coating or a solgel coating. The coatingsand thermal cycles are compatible with the titanium alloy and also withthe braze alloy. Protection of one, but not the other, would be of nobenefit and separate coatings for each are not technically oreconomically desirable due to the complexity of the heat exchangersshape and the complex transitions from bare to braze clad titanium. Notonly do the titanium aluminide and solgel coatings provide oxidationprotection, but they are able to withstand the different thermalstresses due to thermal cycling set up by either titanium or braze cladtitanium, because of their good bonding to the base titanium and braze,their high strength and because they are thin. The braze and titaniumhave different coefficients of thermal expansion so locally at thejunction between braze clad and titanium the coating may be subject tohigh strains and stresses

[0018] Moreover, the coatings maintain adequate strength and ductilityin the titanium, which allows the heat exchanger to handle structuralforces occurring in high temperature heat transfer applications. In anaircraft, for example, there are high pressures and temperature cyclingand vibrations from the surrounding environment (e.g. the aircraftengine on which the heat exchanger is attached). The coatings do notreduce the thermal conductivity of the titanium and, therefore, do notreduce the heat exchanger efficiency.

[0019] Referring to FIG. 2a, an exemplary method of creating a titaniumaluminide protective coating will now be described. An aluminumconversion coating is applied to the heat exchanger to ensure coverageof all of the exposed surfaces (block 210). The conversion coating maybe applied by a mechanical process, such as roll bonding, or by physicalvapor deposition (PVD) or low temperature chemical vapor deposition(LTCVD). Preferably the deposited conversion layer has thickness in therange of 0.5-40 microns.

[0020] The preferred technique, however, is applying the aluminumconversion coating as a gaseous phase at a temperature below whichaluminum does not appreciably react with titanium or braze melts (about500° C.). The gaseous deposition may be performed at temperatures below300° C. An advantage of using gas as a carrier is that the gas flowsthroughout the heat exchanger at a low pressure and velocity. As aresult, heat exchanger surfaces designed to create turbulent gas flow,surfaces at which have stagnant flow areas, and other hard-to-reachsurfaces within the heat exchanger are all uniformly coated. Thus thegaseous deposition results in a conversion coating that is applied witha uniform thickness, even if the heat exchanger has a complex geometry.

[0021] The coated assembly is then heat treated (e.g., in a vacuumfurnace) to activate the conversion coating and transform the aluminuminto titanium aluminide with an alumina surface (block 212). Upon heattreatment, the aluminum oxidizes to form alumina and interacts with thetitanium to form the titanium aluminide. The heating and cooling ratesare controlled to avoid cracking the titanium aluminide. Cracking of thetitanium aluminide coating is a particular concern, as it will result inoxidation of the titanium in the cracked area. As oxygen diffusesquickly in titanium, a single crack can result in the oxidation of acomplex heat exchanger part.

[0022] The aluminum conversion layer may be transformed to titaniumaluminide by heating at a slow controlled rate above 500° C. up to 750°C., followed by a short hold, and cooling at a controlled slow rate downto about 500° C. For example, heating may be at 100° C. per hour after500° C. followed by a 30 minutes hold at 750° C. and then cooling willalso be around 100° C. per hour down to 500° C.

[0023] The resulting surface structure of the titanium metal componentis illustrated in FIG. 3. The diffusion of aluminum and transformationof titanium to titanium aluminide preferably occurs at temperaturesaround 700° C. A conversion layer having thickness in the range of0.5-40 microns could produce a titanium aluminide coating having athickness between 1 and 80 microns.

[0024] After the low temperature heating, a surface layer of aluminaremains on the titanium aluminide. The alumina surface layer alsoprovides oxidation protection. The thickness of the alumina surfacelayer may be between 0.5 microns and 5 microns.

[0025] Correct heat treatment of the protective coating results in anoxidation resistant coating that protects the titanium from oxidationand embrittlement even after exposure for 4,000 hours up to 800° C. Thisis illustrated in Table 1, which indicates mechanical properties ofcoated titanium alloy Ti21S, and a simple alloy Ti 3-2.5, which ishighly prone to oxidation. Foil of seven mil thick was used. This foilis more prone to oxidation than large heat exchanger sections due to itsvery high surface area to volume ratio. TABLE 1 Mechanical Properties ofTi21S & Ti3-2.5 7 mil foil after exposure to high temperatures and theassociated weight gain due to oxidation Weight Temp. Time YS UTSElongation Gain Alloy + Coating [° C.] [hrs] [ksi] [ksi] [%] [μg/cm²]Ti3-2.5 + Al 700 4,000  41  86 14 — Ti3-2.5 + Al 700 4,000  65  83 17 —Ti21S + Al 700 4,000 115 121 4 — Ti21S + Al 700 4,000 117 118 5 — Ti21S700   192  70  75 0.5 1160 Ti21S + Al 700   192 117 130 10.7  50 Ti21S760   192 —  75 ˜0.0 2600 Ti21S + Al 760   192 109 111 13.6 100

[0026] Table 1 shows the mechanical properties of aluminum-coated Ti21Safter exposure to about 700° C. and 760° C. for 192 hours and 4000hours.

[0027] Because the preferred gaseous deposition technique is performedat low temperatures, it has several advantages over conventionaltechniques such as PVD, CVD and in-the-pack and above-the-packtechniques. The conventional techniques involve temperatures (typicallyabout 1,000° C.) that would destroy the mechanical properties of thetitanium due to grain growth or over-aging, depending on the temperatureand alloy The conventional techniques would also open up the possibilityof oxidation of unprotected titanium, even if the process occurs in avacuum. In addition, conventional techniques involve temperatures aboveor close to that of the melt temperature of titanium brazes (typicallyabout 900° C.). Erosion is a common problem with titanium; it isindicative of too high a braze temperature or too long a time at hightemperatures. Even temperatures of about 800° C. could lead to anon-optimum coating and result in erosion and metallurgical problems dueto excessive diffusion. In addition, excessive distortion of the complexshape of the heat exchanger would occur, with leaks forming betweenpassages.

[0028] Deposition of the aluminum at too high a temperature prevents theformation of the surface alumina layer. Deposition at the elevatedtemperature results in the diffusion of the aluminum into the titaniumduring the coating process so afterwards there is no aluminum to oxidizeto alumina. Hence, protection would only be provided by titaniumaluminide and not by the titanium aluminide and alumina layer. For thebraze clad material, dissolution of the aluminum into the braze would becomplete and an oxidation protection barrier would not be formed.

[0029] The heat treatment temperatures determine the type of titaniumaluminides formed and the degree of diffusion of the aluminum into thetitanium. The gaseous coating and heat treating steps can be separatedto give better control of microstructure and the formation of thealumina layer, unlike conventional processes (in-the-pack and above-thepack processes) that combine the coating and heat treating steps.

[0030] Even exposure of aluminum-coated titanium to these hightemperatures results in the dissolution of the aluminum plus theformation of undesirable titanium aluminides as Table 2 shows. Thetitanium alloy Ti21S coated with a thin 10 micron aluminum layer washeat treated over a wide temperature range to illustrate the differentaluminides that can be formed. At the higher temperatures, the phasesare titanium-rich as the aluminum diffuses into the center of thecomponent. The amount of alumina (Al₂O₃) decreased by over a factor offive, going from a major constituent to minor as the heat treatmenttemperature increases. Both of these effects would be expected to reduceoxidation resistance.

[0031]FIG. 4 shows that the weight gain after exposure to 600° C. for192 hrs is the same for the material heat treated at 1,000° C. as forbare material (see box insert) with no protection. While heat treatingat 700° C. results in a low weight gain. TABLE 2 Comparison of TitaniumAluminide Phases formed and Wt. % of Al₂O₃ Vs. Heat Treating TemperatureTemperature [° C.] Titanium aluminide phases Approx. wt % of Al₂O₃ 700Al₃Ti trace Ti 1   850 Al₃Ti, Al₂Ti, AlTi & AlTi₂ 0.5 1,000   AlTi₃,trace Al₂Ti4C₂, & Al₅Ti₂ 0.2

[0032] Referring to FIG. 2b, an exemplary method of creating a solgeldip coating will now be described. The solgel dip coating can be applieddirectly to the titanium or titanium aluminide surface of the heatexchanger by a paintbrush, by spraying or by dip depositing (310).Preferably, the heat exchanger is dipped in a solgel solution for a timeperiod just sufficient to ensure wetting of all the surfaces and removalof any entrapped air. Viscosity of the solgel solution is less than 1centipoise; otherwise the solution may not enter all the passages of theheat exchanger and will block the passages that it does enter. The lowviscosity of less than 1 centipoise also allows for a uniform coatingthickness, which reduces cracking of the coating. Withdrawing the heatexchanger at a controlled, predetermined rate also allows for a uniformcoating thickness. Increasing the rate will increase the coatingthickness. A rate of around 150 mm per min has been found to besatisfactory.

[0033] The solgel coating is formed by a reaction of the solgel solutionwith water usually in the form of moisture in the atmosphere. Thesereactions occur at room temperature. The coating on the heat exchangeris hydrolyzed and subsequently polymerized in a controlled humidityenvironment (312).

[0034] The coating may be rapidly heated and cooled in an air furnaceduring application (314). The heating and cooling allow the solgel to beformed in less than an hour (as opposed to one day, which is typical).For example, the solgel coating may be fired by placing it directly in apre-heated furnace and removed after a roughly 30 min hold. The lowtemperatures avoid the use of a vacuum furnace for each “drying” stepand thereby significantly reduce the cost of applying the solgelprotective coating.

[0035] The solgel coating is heat treated at low temperatures (316). Theheat treatment densities the coating so that it provides a barrier tooxidation. Low temperatures of about 300° C. to 500° C. avoid oxidationof the uncoated titanium. Temperatures over 700° C., in contrast, wouldoxidize the titanium, rendering the heat exchanger brittle and unusable.These high temperatures will result in grain growth and will alsooverage titanium alloys. They will also result in distortion of thecomplex shape of the heat exchanger and leakage between passageways 13and 14.

[0036] The solgel coating is preferably applied through multiple dips.For example, the solgel coating may be applied in three dips wherein thecoatings of the first and second dips are fired (that is, dried) at alow temperature of 300° C. to 500° C. (a temperature at which thetitanium has minimum or no oxidization) for a 30 minute time period; andthe third dip is fired in air or a protective atmosphere at 350° C. to600° C. for a 30 minute time period with no oxidation of the titanium.This rapid firing in air is a marked advantage over the typical 700° C.to 800° C. firing. It makes the process less expensive and more viablebecause it avoids the use of a vacuum furnace that would be required forthe conventional process and reduces time compared to the conventionalprocess.

[0037] Solgel coatings include alumina, zirconia, titanium oxide andsilica. These solgel coatings can be readily applied directly to thetitanium alloy, the braze or to the titanium aluminide coating toprovide oxidation protection. The zirconia and alumina coatings have theadvantage of the rapid firing cycle at a low temperature of 350° C. to600° C. Alumina has the advantage of a higher thermal conductivity thanzirconia.

[0038] These solgel coatings also provide oxidation protection and allowhigh strength and ductility to be retained in the titanium. This is trueeven for a 4 mil thick foil of Ti 3-2.5, an alloy very prone tooxidation, after exposure to 600° C. for 192 hrs. See Table 3. TABLE 3Mechanical Properties of Zirconia Solgel Coated Ti3-2.5 After Exposureto 600° C. for 192 Hours YS UTS El Zirconia solgel [ksi] [ksi] [%]Ti3-2.5 80 103 8

[0039] The titanium aluminide and solgel coatings may be applied tostructures other than heat exchangers. For instance, the coatings may beapplied to exhaust manifolds on internal combustion engines and gasturbine engines, titanium impellers, etc.

[0040] Although the present invention has been described above withreference to specific embodiments, it is not so limited. Instead, thepresent invention is construed according to the claims that follow.

1. A method of fabricating a heat exchanger, the method comprising:forming a titanium heat exchanger core; and applying a protectivecoating to bare titanium and braze surfaces of the core, the coatingbeing applied by applying an aluminum conversion layer to the core at atemperature below which aluminum does not appreciably react withtitanium, and then heat treating the conversion layer so the aluminumoxidizes and interacts with the titanium to form titanium aluminide. 2.The method of claim 1 , wherein the aluminum conversion layer istransformed to titanium aluminide by heating at a slow controlled rateabove about 500° C. followed by a short hold at a temperature no morethan 750° C., and cooling at a controlled slow rate back down to about500° C.
 3. The method of claim 1 , wherein the conversion layer isapplied by gaseous deposition.
 4. The method of claim 3 , wherein thegaseous deposition and heat treating are performed separately.
 5. Themethod of claim 1 , wherein the conversion layer is applied attemperatures below 300° C.
 6. The method of claim 1 , wherein thetitanium aluminide coating has thickness in the range of at least 0.5microns.
 7. The method of claim 1 , wherein the conversion layer isoxidized to form an alumina surface layer.
 8. The method of claim 1 ,further comprising securing manifolds to the core and applying theprotective coating to the manifolds.
 9. The method of claim 1 , furthercomprising applying an outer solgel coating.
 10. A method of fabricatinga heat exchanger, the method comprising: forming a heat exchanger core;applying a solgel solution to the heat exchanger core, the solutionhaving a viscosity of less than I centipoise, whereby a coating results;and sintering the coating at temperatures between about 300° C. to 650°C .
 11. The method of claim 10 , further comprising rapidly heating andcooling the coating after it is applied to the heat exchanger core. 12.The method of claim 10 , wherein the solution is applied in multipledips and dried after each dip, a first dip coating being dried at about300° C.
 13. The method of claim 12 , wherein a second dip is also driedat about 300° C. and a final dip is dried at about 600° C.
 14. Themethod of claim 10 , wherein the solgel solution is applied by dippingthe core in the solution and withdrawing the core at a controlled rate.15. The method of claim 10 , wherein the solgel coating is a zirconiasolgel coating.
 16. A method of applying a coating to a titaniumarticle, the method comprising: applying an aluminum conversion layer tothe article by gaseous deposition, the layer deposited at a temperaturebelow which aluminum does not appreciably react with titanium; and heattreating the conversion layer so the aluminum oxidizes and interactswith the titanium to form titanium aluminide.
 17. The method of claim 16, further comprising applying an outer solgel coating.
 18. A method ofapplying a coating to a titanium article, the method comprising:applying a solgel solution to the article, the solution having aviscosity of less than 1 centipoise, whereby a coating is formed on thearticle; and sintering the coating at temperatures between about 300° C.and 600° C.
 19. A heat exchanger comprising: a core includingtitanium-based components that are bonded together by braze; and aprotective coating on bare metal and braze surfaces of the core, thecoating including a titanium aluminide conversion layer and an aluminatop layer, the conversion layer having a thickness of at least 0.5microns.
 20. The heat exchanger of claim 19 , wherein the titanium-basedcomponents are made of metal selected from the group consisting of CPTi, Ti 3-2.5, Ti 1100, Ti 15-3, Ti21 S, Ti 6242, and IMI
 834. 21. Theheat exchanger of claim 19 , wherein the braze surfaces cover asubstantial portion of the core.
 22. The heat exchanger of claim 19 ,further comprising a solgel coating on the protective coating.
 23. Theheat exchanger of claim 19 , further comprising titanium manifoldsconnected to the core, the manifolds also coated with the protectivecoating.
 24. A heat exchanger comprising: a core includingtitanium-based components that are bonded together by braze, and asolgel coating on bare metal and braze surfaces of the core.
 25. Anarticle comprising: a titanium-based member; and a braze on the member;an outer unbrazed portion of the member being made of titaniumaluminide; an outer portion of the braze being made of an aluminide. 26.The article of claim 25 , wherein the braze is titanium-based.
 27. Anarticle comprising: a titanium-based substrate; a braze on thesubstrate; and a solgel coating over the substrate and the braze; thesolgel coating being in direct contact with the substrate and the braze.28. The article of claim 27 , wherein the solgel coating has a thicknessof no more than 5 microns.